Gigabit ethernet transceiver with analog front end

ABSTRACT

Circuitry to remove switches from signal paths in integrated circuit programmable gain attenuators. Programmable gain attenuators and programmable gain amplifiers commonly switch between signal levels using semi-conductor switches. Such switches may introduce non-linearities in the signal. By isolating the switches from the signal path linearity of the PGA can be improved.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present Application is a continuation of U.S. application Ser. No.10/695,313, filed Oct. 28, 2003, now U.S. Pat. No. 7,038,533, which is adivisional of U.S. application Ser. No. 10/282,688, filed Oct. 29, 2002,now U.S. Pat. No. 7,671,681, which is a continuation of application Ser.No. 09/712,422, filed Nov. 13, 2000, now U.S. Pat. No. 6,472,940, whichclaims priority from Provisional Application Nos. 60/164,970,60/164,980, and 60/164,981, filed Nov. 11, 1999, and 60/181,989, filedFeb. 11, 2000.

FIELD OF THE INVENTION

The invention relates to analog circuits in an integrated circuitenvironment and, in particular embodiments, to low voltage integratedcircuits having both digital and analog components.

BACKGROUND OF THE INVENTION

As higher levels of circuit integration are achieved more analogfunctions are being mixed with digital functions on the same integratedcircuit. In addition, as circuit dimensions shrink, integrated circuitsupply voltages decrease. There is therefore a need in the art fortechniques to facilitate the use of lower voltages in mixed integratedcircuits.

SUMMARY OF THE INVENTION

The present invention discloses methods for providing a control signalfor a programmable gain attenuator. First an initial value for a finecontrol portion of a programmable gain attenuator is set. An AGC portionof a control loop then compares an average absolute value of the signalcontrolled to a reference value. Once the value of the course AGC hasconverged sufficiently, the operation of the coarse AGC portion of thecontrol loop is terminated and the operation of the fine control portionof the programmable attenuator is begun.

The invention discloses apparatus for providing an adjustable bandwidthhigh pass filter while minimizing the effect on signal amplitude. Ahighpass filter has an input capacitor in series with a resistiveladder. The resistive ladder comprises a plurality of resistors coupledin series. The coupling between the capacitor and the first resistor ofthe resistive ladder defines a first tap and successive couplingsbetween resistors form successive taps. The last resistor of saidresistive ladder is coupled to a ground. A plurality of bandwidthadjusting resistors, each having a first side coupled to the first tap,are included. A plurality of switches, each providing coupling for asecond side or a corresponding bandwidth adjusting resistor to saidground are also included.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the accompanying drawings in which consistent numbersrefer to similar parts throughout:

FIG. 1A is a graphic illustration of an environment in which embodimentsof the invention may operate.

FIG. 1B is a block diagram of an exemplary embodiment of the invention.

FIG. 2 is a simplified block diagram of the functional architecture andinternal construction of an exemplary transceiver block.

FIG. 3 is a block diagram of an analog section or an exemplary gigabitreceiver.

FIG. 4 is a schematic diagram of a programmable gain attenuator (PGA).

FIG. 5 is a schematic diagram, according to an embodiment of the presentinvention.

FIG. 6 is a schematic diagram, according to an embodiment of the presentinvention.

FIG. 7 is a graphical illustration of a multiple switch (and multipletap) programmable gain attenuator (PGA).

FIG. 8 is a schematic diagram illustrating a multi slice variant of aprogrammable gain attenuator, according to an embodiment of theinvention.

FIG. 9 is a schematic diagram of a exemplary prior art PGA.

FIG. 10 is a schematic diagram illustrating exemplary prior artcircuitry.

FIG. 11 is a schematic diagram according to an embodiment of the presentinvention.

FIG. 12 is a schematic diagram of an exemplary embodiment of the presentinvention.

FIG. 13 is a schematic diagram of a PGA, in which switches have beenremoved from the signal path.

FIG. 14 is a schematic diagram of a PGA having N taps.

FIG. 15 is a schematic of a high-pass filter combined with a PGA.

FIG. 16 is a schematic diagram of a circuit which may be used to adjusta corner frequency of a high-pass filter PGA combination (HPGA).

FIG. 17 is a combination schematic and block diagram of a circuit usedto change the corner frequency of a HPGA, without affecting the voltagesteps available at the taps of the HPGA.

FIG. 18 is a schematic diagram of a circuit used to change a cornerfrequency of a HPGA without affecting the voltage steps available at thetaps of the HPGA.

FIG. 19 is a schematic diagram of a HPGA illustrating the switchingdevice circuitry according to an embodiment of the invention.

FIG. 20 is a schematic diagram of circuits combined with a HPGA.

FIG. 21 is a schematic diagram illustrating an exemplary PGA.

FIG. 22 is a schematic diagram of an exemplary embodiment of the currentinvention illustrating a sliding window control circuit.

FIG. 23 is a schematic diagram illustrating an example of the slidingwindow concept applied to an “R to R” resistance ladder.

FIG. 24 is a schematic diagram illustrating an embodiment of theinvention, in which interpolation resistors have been added.

FIG. 25 is a schematic diagram of circuitry, as may be used to implementa sliding window switch control.

FIG. 26 is a circuit diagram of an eight segment PGA ladder.

FIG. 27 is a schematic diagram of one of the segments as illustrated inFIG. 26.

FIG. 28 is a schematic diagram representing another segment asillustrated in FIG. 26.

FIG. 29 is a graph of the frequency response of a Bandpass Programmablegain attenuator BPGA according to an embodiment of the presentinvention.

FIG. 30 is a graph of an exemplary PGA step size versus the step number.

FIG. 31 is a block diagram further illustrating programmable gainamplifier 214.

FIG. 32 is a graph of the course and fine steps of combined course andfine PGAs.

FIG. 33 is a block diagram of an AGC system as may be used to controlPGAs according to an embodiment of the invention.

FIG. 34 is a chart of Exemplary Peak to RMS values.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A is a graphic illustration of an environment in which embodimentsof the invention may operate. In FIG. 1A a server computer 101 iscoupled to work stations 105A and 105B through a bi-directionalcommunication device, such as a gigabit Ethernet transceiver 107A and107B. The particular exemplary implementation chosen is depicted in FIG.1B, which is a simplified block diagram of a multi-pair communicationsystem operating in conformance with the IEEE (Institute of Electricaland Electronic Engineers) 802.3AB standard for 1, gigabit per second(GB/S) Ethernet full-duplex communication over 4 twisted-pairs ofcategory-5 copper wires. Gigabit transceiver 107A is coupled to workstation 105A via a communication line 103A. The communication line 103Aincludes 4 twisted-pair of category-5 copper wires. Communications line103A is coupled to a bi-directional gigabit Ethernet transceiver 107Cwhich may be identical to the bi-directional gigabit Ethernettransceiver 107A. Similarly, server 101 also communicates through agigabit Ethernet transceiver 107B using a 4 twisted-pair set ofcategory-5 copper wires 103B coupled to an Ethernet transceiver 107D,which is further coupled to work station 105B. The work stations may befurther coupled to other Ethernet devices. The server 101 may also becoupled via an Ethernet device 107E and to other devices, such asservers or computer networks.

FIG. 1B is a block diagram of an exemplary embodiment of the inventionwithin the communication system illustrated in FIG. 1A. Thecommunication system illustrated in FIG. 1B is represented as apoint-to-point system, in order to simplify the explanation, andincludes two main transceiver blocks 107A and 107C, coupled togetherwith 4 twisted-pair cables. Each of the wire pairs 112A, B, C, and D iscoupled between the transceiver blocks 107A and 107C through arespective 1 of 4 line interface circuits 106. Each line interfacecircuit communicates information developed by respective ones of 4transmitter/receiver circuits (constituent transceivers) 108 coupledbetween respective interface circuits and a physical coding sublayer(PCS) block 110. Four constituent transceivers 108 are capable ofoperating simultaneously at 250 megabits per second (MB/S), and arecoupled through respective interface circuits to facilitate full-duplexbi-directional operation. Thus, one GB/S communication throughput ofeach of the transceiver blocks 107A and 107C is achieved by using 4 250MB/S (125 megabaud at 2 bits per symbol) constituent receivers 108 foreach one of the transceiver blocks and 4 twisted-pairs of copper cablesto connect the two transceivers together.

FIG. 2 is a simplified block diagram of the functional architecture andinternal construction of an exemplary transceiver block, indicatedgenerally at 200, such as transceiver 107A. Since the illustratedtransceiver application relates to gigabit Ethernet transmission, thetransceiver will be referred to as the “gigabit transceiver.” For easeof illustration and description, FIG. 2 only shows one of the four 250MB/S constituent transceivers which are operating simultaneously (termedherein 4-D operation). However, since the operation of the fourconstituent receivers are necessarily interrelated, certain blocks inthe signals lines in the exemplary embodiment of FIG. 2 perform andcarry four-dimensional (4-D) functions and 4-D signals, respectively. By4-D it is meant that the data from the four constituent receivers areused simultaneously. In order to clarify signal relationships in FIG. 2and FIG. 1B, single lines which correspond to more than one line arerepresented by a slash followed by a number. The slash followed by thenumber indicates the number of lines represented by the singleillustrated line.

With reference to FIG. 2, the gigabit transceiver 200 includes a GigabitMedium Independent Interface block 202, a Physical Coding Sublayer (PCSblock) 204, a pulse-shaping interface block 210, a high-pass filter 212,a programmable gain amplifier (PGA) 214, an analog-to-digital (A/D)converter 216, an automatic gain control block 220, a timing recoveryblock 222, a pair swapping multiplexer block 224, a demodulator 226, anoffset canceler 228, a near-end cross talk (NEXT) canceler block 230having three NEXT cancelers, and an echo canceler 232. The gigabittransceiver 200 also includes AID first-in-first-out buffer (FIFO) 218to facilitate proper transfer of data from the analog clock region tothe received clock region, and a FIFO block 234 to facilitate propertransfer of data from the transmit clock region to the receive clockregion. The gigabit transceiver 200 can optionally include a filter tocancel far and cross-talk noise (FEXT canceler).

On the receive path, the line interface block 210 receives an analogsignal from the twisted pair cable. The received analog signal ispreconditioned by high-pass filter 212 and a programmable gain amplifier(PGA) 214 before being converted to a digital signal by the AIDconverter 216 operating at a sampling rate of 125 MHZ. Sample timing ofthe AID converter 216 is controlled by the output of a timing recoveryblock 222 controlled, in turn, by decision and error signals from ademodulator 226. The resulting digital signal is transferred from theanalog clock region to the received clock region by an AID FIFO 218, anoutput of which is also used by an automatic gain control circuit 220 tocontrol the operation of the PGA 214.

FIG. 3 is a block diagram of the analog section of an exemplary gigabitreceiver. In FIG. 3, line interface 210 receives data from a twistedpair, which comprises one-quarter of the gigabit receiver interface. Thedata received by the line receiver is then coupled into a high passfilter 212, which filters the data and further couples it to theprogrammable gain amplifier 214. The programmable gain amplifier 214includes sections: a coarse programmable gain attenuator 16 and a fineprogrammable gain attenuator 14. The signal, which is input to the PGA,is first attenuated by the coarse PGA 16 and then the signal is providedto the fine PGA 14. Because the signal levels for the coarse PGA arehigher than the signal levels of the fine PGA, different designs foreach may be employed. The fine PGA 14 provides an attenuated signal to aconverter 216. The A/D converter 216 accepts the attenuated signal fromthe fine PGA 14, digitizes it, and provides the digitized signal to anA/D FIFO 218. An automatic gain control (AGC) 220 examines the values inthe A/D FIFO 218 and then provides adjustment to the PGA 214. Theautomatic gain control 220 adjusts the coarse PGA (16) by using afour-bit digital signal. The automatic gain control also controls thefine PGA (14) using a five-bit digital signal.

The PGA 214 can be a programmable gain attenuator or it may be coupledto a fixed at variable amplifier to form a programmable gain amplifier,as the structure of the PGA is compatible with either. The distinctionis somewhat academic as a programmable gain amplifier may amplify asignal by less than 1. Therefore both the programmable gain amplifierand programmable gain attenuator shall be referred to hereafter as aprogrammable gain amplifier (PGA).

Programmable gain attenuators commonly employ switches in order tochange between gain settings. These switches are commonly semiconductorintegrated switches, which may cause problems because of nonlinearities,capacitances, and other characteristics inherent in the switches. InFIGS. 4 through 8 these problems and embodiments of solutions arediscussed.

FIG. 4 is a schematic diagram of an illustrative prior art programmablegain attenuator. In FIG. 4, an input signal is accepted at the input 401to buffer 403. The buffered signal is then divided among resistors R405,R407 and R417, according to their resistance values. Common mode voltage419 provides a DC bias for signals being coupled into the input ofbuffer 413.

By selecting either switch 409 or switch 411, different attenuations areselected, resulting in a different signal being coupled into buffer 413.Buffer 413 may be a fixed amplifier, thereby providing an output signalmultiplied by the gain of the buffer 413. If switch 409 is closed, thevoltage tap, defined by the junction of resistor R405 and resistor R407,will provide the input to buffer 413. However, if switch 411 is closed,the voltage appearing at the voltage tap defined by the junction of R407and R417 will be provided to buffer 413. By selecting either switch 409or 411, a variable gain can be programmed into the PGA circuit of FIG.4.

There are, however, problems with the circuit arrangement illustrated inFIG. 4. One problem is that no matter which tap is selected, a switch isdirectly in the signal path. Accordingly, the signal is affected bycharacteristics of the switch. Because the switch is not a perfectswitch it has a finite nonlinear resistance. The resistance of theswitch forms a voltage divider with the input impedance of buffer 413.The voltage divider changes the signal level to the input of buffer 413.To decrease the influence of switches 409 and 411 on the voltageprovided to the input of buffer 413, it is desirable to make switchresistance as low as possible. Making switches 409 and 411 physicallylarger will decrease switch resistance. As a switch is enlarged thecapacitance of the switch increases. As the capacitance of the switchincreases, the bandwidth of the PGA decreases. Therefore, there is atradeoff between switch resistance and bandwidth. In addition, becauseof the common mode voltages and the low power supply voltage commonlyavailable in mixed analog and digital ICs, it may be difficult toprovide the necessary drive, with the available voltages, to insure goodcapacitance. In other words, it may be difficult to assure completeconduction of the switch with the control voltages available to controlthe switch.

FIG. 5 is a schematic diagram according to an embodiment of the presentinvention. In FIG. 5, each voltage divider tap has an individualtermination resistor. In the PGA of FIG. 5, the signal to be amplifiedis coupled into buffer 503 through input 501. If switch 513 is on andswitch 515 is off, the tap voltage at the junction of R505 and R509 iscoupled through resistor 507 into buffer 517. Because the impedance ofbuffer 517 is generally very high, the amount of current flowing throughR507 is typically negligible. If switch 515 is on the tap voltage at thejunction of resistor R507 and resistor R511 appears as the input tobuffer 517, resistor R509 is chosen so that switch 513 is of negligibleresistance when compared with R509. (Similarly, switch 515 is ofnegligible resistance when compared with resistor R511.) Additionally,since the signal path is always from buffer 503 through resistor R505and resistor R507 into buffer 517, neither switch is in the signal path,and the effects of switch nonlinearities and switch capacitances areminimized. Even if switch 513 and switch 515 are nonlinear, thenonlinear resistances provided by those switches are small compared withresistor R509 or resistor R511. Because the nonlinear resistance of theswitches is small compared to the termination resistors R509 and R511,the overall contribution of the nonlinear resistance of the switches onthe voltage, which is coupled into buffer 517 is small. Additionally,any switch capacitance is isolated from the signal path by terminationresistors R509 and R511. In contrast, in the circuit of FIG. 4, thesignal to be amplified travels through the switches and the switchcapacitances add in parallel. Therefore, if an architecture similar toFIG. 4 is used, the capacitance of the switches tend to accumulate asadditional switches are added. In contrast, in FIG. 5 the capacitancesof the switches are isolated from the signal path by terminationresistors R509 and R511. As a consequence, control switches, such asthose illustrated in FIG. 4, may be constrained to be of thetransmission type, comprising PMOS and NMOS devices which are moreconductive but are more complicated to use than NMOS (Metal OxideSemiconductor) switches. A simple NMOS switch, however, may be used withthe arrangement illustrated in FIG. 5.

FIG. 6 is a schematic diagram similar to FIG. 5 except that the idealswitches illustrated in FIG. 5 have been replaced by NMOS switchingdevices. In FIG. 6, the signal to be amplified is provided to input 601of buffer 603. If device U613 is turned on and U615 is turned off, thenthe voltage divider comprises R605 and R619. If U615 is turned on andU613 is turned off, then a divider is formed from resistors R605, R607and R621. Because R619 and R621 isolate the switching devices U613 andU615 from the signal path, the on-state resistance of U613 and U615 areof less consequence than the on-state resistance of the switches in thecircuitry of FIG. 4. That is, instead of having to use the “expensive”transmission type switch, as would be the case in embodiments using thearrangement of FIG. 4, devices such as U613 and U615 can be made. Forexample, U613 and U615 can be simple NMOS (Negative Metal OxideSemiconductor) type devices. The size of U613 and U615 is dictated, inpart, by the resistance of R619 and R621, which isolate U613 and U615from the signal path.

FIG. 7 is a graphical illustration of a multiple switch and multiple tapPGA. In the diagram of FIG. 7, the signal to be amplified is provided tobuffer 703 through input 701. The signal travels through a resistornetwork represented by R705, R707 and R709. Any number of resistors canbe included between R707 and R709. At each resistor junction (tap), aterminating resistor such as terminating resistor R717, is inserted.When one switch is turned on, and the other switches are turned off, theterminating resistor connected to that switch forms a voltage dividerwith the resistor network, thereby providing a divided input to buffer711. As illustrated in FIG. 7, any number of switches and terminatingresistors can be accommodated. It is possible to add multiple switchespartly because each switch is isolated from the signal path instead ofthe signal having to travel through the switch as in FIG. 4. Inaddition, the capacitances of the switches in FIG. 7 have much lesseffect on the signal being amplified than the switches illustrated inFIG. 4. The terminating resistors in FIG. 7 isolate the switch'scapacitance from the signal path, thereby allowing more switches to beadded, with less effect on the bandwidth of the PGA.

FIG. 8 is a schematic diagram illustrating a multi-slice variant of aprogrammable gain attenuator, according to an embodiment of theinvention. In the Embodiment illustrated in FIG. 8, the PGA is segmentedinto slices. An interpolation resistor R823 may then be applied inparallel with each slice. Interpolation resistor R823 helps reduce theratio between, for example, R805 and R811; or R805, R807 and R813; orR805, R807, R809 and R815, etc. By reducing the ratio necessary betweeninline resistors such as R805 and R807, radically different resistorvalues are not required for a certain step attenuation setting (forexample, 1 dB/step). Any number of slices may be joined together inseries in order to implement a programmable gain attenuator.

If the signal provided to the PGA is large enough to cause the absolutevoltages on certain nodes within the PGA to exceed the power supplyvoltage, distortion can result. A common configuration for the PGA whichmay show such distortion is illustrated in FIG. 9.

Programmable gain attenuators are commonly employed in integratedcircuits having low voltage supplies. The low voltage supply can causeproblems when large input signals are coupled into PGAs. In FIGS. 9through 10 examples of such problems are illustrated. In FIGS. 11through 14 embodiments which deal with such types of problems arediscussed.

FIG. 9 is a schematic diagram of an exemplary prior art PGA. An inputsignal at input 901 is coupled by capacitor C903 into the PGA circuit.Voltage source 919 provides any common mode voltage which is needed bybuffer 909. The input signal is divided through the resistive laddercomprising resistors R915 and R917. The desired signal level can betapped from the resistive ladder through the use of switches 905 or 907.If the voltage amplitude of the signal input at 901 is large enough, itmay exceed the power supply voltage, which is used to turn switches 905and 907 off and on. If the input signal plus the common-mode voltage 919exceeds the supply voltage, switch 905 or switch 907 may encounterdifficulties turning on or turning off. So, for example, if a largesignal is provided to input 901 through capacitor C903 to switch 905,the positive portion of the input signal may forward bias switch 905.

Once switch 905 is forward biased, for example by a large amplitudeinput signal, switch 905 will begin to turn on, and a voltage spike maybe coupled into buffer 909. This condition is illustrated more fully inFIG. 10, in which the ideal switches of FIG. 9 are replaced with actualMOS switching devices.

FIG. 10 is a schematic diagram illustrating exemplary prior artcircuitry. In FIG. 10, an input signal is coupled into input 1001. Theinput signal is then further: coupled through capacitor 1003 and intoresistor network R1015 and R1017. If switch U1007 is initially turnedon, the input signal is divided by the voltage divider comprisingresistors R1015 and R1017. The tap voltage at the junction of resistorsR1017 and R1015 is coupled by switch U1007. into buffer 1009. If asignal with a large enough peak voltage enters at input 1001, thevoltage at the source of U1005 may exceed V_(CC) the (supply voltage),which is coupled to the gate of U1005. When the source voltage of deviceU1005 exceeds its gate voltage by an amount approaching threshold,voltage, U1005 will start to turn on. Once device U1005 turns on, theattenuating effect of resistor R1015 on the signal applied to the buffer1019 is eliminated and the input signal is coupled directly into buffer1009. This signal dependent turning on of device U1005 may cause asignificant nonlinearity. Such nonlinearities may be extremelydetrimental to circuit performance. The situation is exacerbated by thefact that the common-mode voltage 1019 may be high, and the power supplyvoltage of modern mixed analog and digital integrated circuits tends tobe low.

FIG. 11 is a schematic diagram according to an embodiment of the presentinvention. Divider circuitry in FIG. 11 is identical to the dividercircuitry in FIG. 10. That is, input 1101 receives an input signal,couples it into a capacitor C1103, which further couples the inputsignal into a voltage divider comprising resistors R1109 and R1115. Thecircuitry of FIG. 11 also comprises a common mode voltage source 1119.However, unlike the circuit of FIG. 10, the tapped output of the voltagedivider R1109 and R1115 is coupled into a buffer amplifier 1113.Similarly, the voltage tap comprising capacitor C1103 and resistor R1109is coupled into a buffer amplifier 1105. The buffer amplifiers 1105 and1113 are controlled by switches 1111 and switch 1117, respectively.Switches 1111 and 1117 essentially provide the operating current foreach buffer amplifier (U1105 and U1113), when the corresponding switchis closed. No operating current to the buffer amplifier is provided whenthe corresponding switch is open. Accordingly, switch 1111 is not in theinput circuit path and is not subject to turn on due to the variationsin input signal. The signal at the junction of C1103 and R1109 iscoupled into the input of buffer 1105. If the voltage at the junction ofC110 3and R1109 exceeds the power supply of voltage and no current isbeing provided to buffer amplifier 1105, nothing happens because bufferamplifier 1105 is not active. Therefore, even when the voltage at thejunction of C1103 and R1109 exceeds the power supply voltage none of thevoltage is coupled through to an output 1107 because the buffer 1105 hasbeen deactivated. Buffer 1105 isolates switch 1111 and the output 1107from large input signals.

FIG. 12 is a schematic diagram of an implementation of the circuitillustrated in FIG. 11. In the circuit of FIG. 12 the ideal switches1111 and 1117 have been replaced by NMOS (Negative Metal OxideSemiconductor) switches. Additionally, buffer amplifier 1105 has beenreplaced by a MOS follower U1207, and buffer amplifier 1113 has beenreplaced by a MOS follower U1211. So, for example, if follower 1211 hasbeen selected by placing a high level control voltage at the gate ofdevice U1213, then the voltage at the output 1221 will reflect thevoltage at the junction of R1205 and R1215. U1213 is turned on byplacing a high voltage on its gate. U1209 may be turned off by groundingits gate. Once the gate of U1209 is coupled to ground, no current canflow through device 1209. If a large voltage spike occurs at thejunction of C1203 and R1205, it will couple to the gate of U1207 (exceptpossibly for a small amount of capacitive coupling). However, thevoltage spike will not be coupled through U1207 because there is nocurrent flowing in the device 1207 (unless the voltage is so high at thegate of 1207 that the actual gate insulation of device U1207 breaksdown).

The configuration of FIG. 12, however, places switches U1209 and U1213in the signal path. Therefore, non-linearities from devices U1209 andU1213 can be introduced into the signal. It is desirable to removeswitching components from signal interaction by removing them from thesignal path.

FIG. 13 is a schematic diagram of a PGA similar to FIG. 12 except thatthe switches have been moved from the signal path by placing them in thedrain circuit of amplifiers U1307 and U1311 rather than in the sourcecircuit. By coupling the gate of either U1305 or U1309 to ground, theamplifier devices U1307 and U1311 are respectively turned off.Therefore, if a large signal is input to 1301 it may couple acrosscapacitor CI301, and thus appear at the gate of U1307. U1307 may attemptto turn on if the input voltage at the junction of C1301 and R1303 ishigh enough. However, if the device 1305 has its gate coupled to ground,this prevents a current from flowing in U1307 regardless of the voltageat its gate. In this manner, by placing the switch device within thedrain of the follower device, the problem of having a large voltageinput turn on the device and the problem of having the switch in thesignal path are both avoided.

FIG. 14 is a schematic diagram of a programmable gain attenuator havingmultiple taps. In FIG. 14, an input signal is coupled into the PGAthrough input 1401. The input signal is then AC coupled across capacitor1403 and into a resistive ladder comprising resistors R1921, R1923,R1925, R1927, and common mode voltage source 1929. Each voltage tap ofthe circuit is connected to a follower device such as U1407, U1411,U1415, or U1419. The switch devices are all placed in the drain circuitof the amplification devices. So, for example, follower device U1407 hasswitch device U1405 in its drain circuit. Similarly, in the final stageof the PGA, switch U1417 is in the drain circuit of amplification deviceU1419. Similarly, multiple taps can be accommodated.

Commonly programmable gain attenuators may be combined with a high passfunction. FIGS. 15 through 20 illustrate problems encountered andembodiments of the present invention which deal with such problems.

FIG. 15 is a schematic of a high-pass filter combined with aprogrammable gain attenuator (HPGA). Due to the large signal levels andlow supply voltages in many mixed analog and digital integrated circuitapplications, a metal-metal or poly-poly capacitor is commonly used forcapacitor C1503 at the input of such a network. Such a capacitor isgenerally not tunable. Also, resistors R1505 and R1507 are not tunable.The high pass corner (3 dB point) can be adjusted by switchingcapacitance or resistance in and out of the circuit. Generally, thepreferred method is to switch resistors in and not capacitors. This isbecause the signal levels at the input of the capacitor may besignificantly larger than elsewhere in the circuit.

In FIG. 15, the high pass corner of the circuit illustrated is formed bya combination of C1503, R1505 and R1507. The high pass corner frequencyis independent of where voltages are tapped (tap 0 or tap 1). The highpass corner is dependent on the input capacitance C1503 and the seriesresistance of R1505 and R1507. The voltage obtained from theprogrammable gain high pass filter is dependent on whether tap 0 or tap1 is employed as the voltage output tap. The corner frequency of theC1503, R1505, R1507 network, however, does not change no matter whichvoltage tap is used. The corner frequency is dependent only on the valueof C1503 and the serial combination R1505 and R1507.

It is also desirable that the changing of the high pass corner frequencynot affect the gain of the programmable gain attenuator portion of thecircuit. Some mechanism for adjusting the high pass corner frequency ofthe circuit without changing the gain of the circuit is needed.

FIG. 16 is a schematic diagram of a circuit which may be used to adjustthe corner frequency by using a switch 1609 to short out resistor R1611.By shorting out R1611 the overall series resistance of the serialcombination of R1605, R1607 and R1611 is changed. Because the resistancein series with capacitor 1603 is changed, the corner frequency ischanged. Shorting out R1611, however, will change the gain that isavailable at tap 0 and tap 1 of the circuit. It is preferable that whenthe corner frequency changes, the gain per tap not change.

FIG. 17 is a schematic diagram of a circuit used to change the cornerfrequency of a HPGA without affecting the voltage steps available at thetaps of the HPGA. In FIG. 17 the corner frequency of the circuit isdetermined by capacitor 1703 and resistors R1705, R1707 and R1711. Thecutoff frequency is determined by the value of capacitor 1703 and theseries combination of resistors R1707 and R1711, in parallel withresistor R1705. By turning on switch 1709 the overall resistance seen inseries with capacitor C1703 is changed, however, the ratio of thevoltages available at tap 0 and tap 1 remains constant because it isdependent only upon the ratio of RI707 to R1711. The circuit of FIG. 17,however, exhibits a problem. Switch 1709 is configured so that, in orderto turn the switch off, it is convenient to couple the gate of switch1709 to the power supply V_(CC). This approach is problematical becausea large signal, coupled to the input 1701, will be communicated acrosscapacitor 1703. When the switch 1709 is turned off the entire voltageseen at the juncture of C1703 and R1705 will be coupled to switch 1709.If the switch 1709 turns on during the high point of a large inputvoltage signal, the corner frequency of the circuit will change asresistor 1705 is switched into the circuit. The corner frequency willthen change back when the input voltage no longer exceeds the turn onvoltage of the switch 1709 (and switch 1709 turns off). Therefore, if asufficiently large input signal is encountered, the corner frequency ofthe circuit may continually change.

FIG. 18 is identical to FIG. 17 except that the ideal switch 1709 hasbeen replaced by a MOS switching device U1811. If the gate of switchingdevice U1811 is coupled to the power supply V_(CC), and the source ofU1811 receives a voltage that is sufficiently higher than V_(CC), deviceU1811 will turn on.

FIG. 19 is a schematic diagram in which the switching device circuitryhas been augmented. In FIG. 19, switch U1911 can be turned off withoutlarge input voltages causing it to turn back on. The gate of switchU1911 is coupled to a long channel triode device U1907. The long channeltriode device may be inserted in lieu of a high resistance resistor. Atri-state buffer 1919 is also coupled to the gate of the switchingdevice U1911. In order to turn the switch U1911 on, the tri-state buffer1919 leaves the tri-state mode and turns on, thereby coupling the gateof U1911 to ground. To turn device U1911 off, tri-state buffer 1919 isturned off and tri-stated. When the tri-state buffer 1919 turns off andis tri-stated, the gate of U1911 is pulled up to V_(CC), the powersupply voltage, and conducted by the long channel triode device U1907.If a large signal is input at 1901, the signal couples through C1903,through resistor R1905, and into C1909. C1909 is coupled between thegate and source of switch device U1911. Because the tri-state buffer1919 and the long channel triode device U1907 are high input impedancedevices, substantially no current can be conducted through capacitorC1909. Because essentially no current is conducted through C1909,voltage coupled to C1909 at the junction of C1909 and the source voltageof U1911 is essentially coupled across C1909, to the gate of U1911,thereby preventing U1911 from turning on by keeping the V_(GS) of deviceU1911 close to 0.

FIG. 20 is a schematic diagram of a HPGA having multiple circuitssimilar to those illustrated in FIG. 19. In FIG. 20 the shunt frequencyadjustment resistors, for example R2005, are controlled by switchingcircuits comprising a long channel triode device U2011, capacitor 2013,and a tri-state buffer 2025. The same arrangement illustrated in FIG. 19is repeated for multiple devices (see FIG. 20), resulting in N differentcorner frequencies. There is a problem, which might be exhibited withinthe circuitry illustrated in FIG. 20. In FIG. 20, the body of the deviceU2015 may be tied to V_(CC). The forward biasing of the bulk junction(which comprises device U2015) may cause non-linearity problems. Asimilar approach to that taken in FIG. 19 may be implemented to correctthe nonlinearity problem. That is, a long channel device similar toU1907 could be connected between V_(CC) and the body of U2015, insteadof tying the body of U2015 directly to V_(CC).

In PGAs in which signals are small compared with the power supplyvoltage, it may be desirable to employ a simple switching scheme asillustrated in the prior art of FIG. 21. In FIGS. 21 through 32embodiments illustrating methods of improving the performance of thistype of PGA are described.

FIG. 21 is a schematic diagram illustrating an exemplary prior artprogrammable PGA gain attenuator. In FIG. 21 a signal is coupled frominput 2101 to buffer 2103. The output of buffer 2103 is coupled into aresistive ladder comprising resistors R2105, R2107, R2109, R2111, R2113,R2115 and R2117 arranged in series. The desired voltage is tapped fromthe resistive ladder through a series of switches 2121, 2123, 2125,2127, 2129 and 2131. The tapped voltage is then coupled into the outputbuffer 2135. This architecture has been discussed previously. If theinput signal to the resistive ladder is large, then problems withswitches turning on erroneously become a concern. However, the circuitillustrated in FIG. 21 may be used in circuits where the signal to bedivided comprises a small peak-to-peak value, thus assuring againsttransient voltages. The circuitry illustrated in FIG. 21 still exhibitsthe problem of signals traversing the switches. If the circuitry in FIG.21 is to be effectively used, then an important consideration is to makethe switch resistance as low as possible so that the voltage divider,comprising the switch resistance and the input impedance to buffer 2135,does not cause undesired changes at the input to buffer 2135.

FIG. 22 is a schematic diagram of a portion of an exemplary embodimentof the current invention. FIG. 22 is similar to FIG. 21, except thatinstead of turning one switch on at a time as illustrated in FIG. 21with switch 2127, in FIG. 22 two switches, 2225 and 2227 are turned onat the same time. The next tap higher turns on switches 2223 and 2225.The next tap lower turns on switches 2227 and 2229. In this way asliding window of two switches is used to couple the output buffer 2223to the resistive ladder. Because two switches are turned on in parallel,the total switch resistance is decreased. This type of sliding windowmechanism may be extended to any number of switches. That is, forexample, a sliding window of three switches for instance turning onswitches 2223, 2225 and 2227 at the same time. A problem with employinga sliding window switching approach is that when the window slidescompletely towards one or another of the resistive ladder, there is onlyone switch available. Therefore, in embodiments of the invention inwhich it is desirable to keep the same attenuation step between taps,additional switches can be added to either end of the divider ladder. Anumber of switches may be added to either end so that the end switchresistance matches the average switch resistance anywhere within thesliding window of switches.

FIG. 23 is an example of the sliding window concept applied to an “R toR” resistance ladder. An “R to R” ladder as illustrated in FIG. 23 maybe used to make the steps between taps logarithmic (on a linear DBscale). In contrast the resistive ladder in FIG. 22 can be used tomaintain a linear step between taps. In order to obtain logarithmicsteps with a configuration as shown in FIG. 22 without the “R to R”ladder, the resistor values may have to vary by across a larger rangethan is practical within integrated circuits.

FIG. 24 is a schematic diagram illustrating an embodiment of theinvention in which interpolation resistors are added. In FIG. 24,interpolation resistors R2413, R2419 and R2427 have been added dividingthe resistive ladder into multiple segments. By placing an interpolationresistor such as R2413 between two segments, the ratio necessary betweenthe ladder resistors, for example, R2407, R2409 and shunt resistorR2417, can be minimized. If the termination resistor such as R2417 wereto get too large in comparison with the resistive ladder resistors, suchas R2407 and R2409, there may be implementation problems in obtainingthe proper matching ratio in resistors with such disparate values.

FIG. 25 is a schematic diagram of circuitry as may be used to implementa sliding switch window control. One difficulty with implementingcontrols for sliding ladders is that, as the number of switch tapsincreases, so does the amount of control logic that is necessary tocontrol the taps.

The circuit of FIG. 25 may be used to control a sliding window ofswitches. The basic logic comprises a daisy chained set of OR gatesequal to the number of switches to be controlled. In the illustration inFIG. 25, OR gates number N through N+6 are illustrated. Each OR gate hastwo inputs. The first input is coupled to the output of the preceding ORgate in a daisy chain fashion. That is, OR gate N+2 has one input whichis coupled to the output of OR gate N+1. OR gate N+1 has output of ORgate N as an input. The other input to the OR gate serves as a controlsignal input. Additionally, the output of each OR gate is coupled to theinput of a companion Exclusive OR gate. That is, the output of the NthOR gate 2501 is coupled to the input of the Nth Exclusive-OR 2503.Similarly, the N+1 OR gate 2505 has its output coupled to the input ofthe N+1 exclusive OR gate 2507. In other words, each OR gate has acompanion Exclusive-OR(exor) gate. The companion Exclusive-OR gateaccepts an input from its companion OR gate as illustrated in FIG. 25.The second input to the exclusive OR gate is coupled to the output of anOR gate, which is further up the daisy chain. The distance between ORgates whose outputs are coupled to the inputs of the exclusive ORdetermines the size of the sliding window. In the illustration in FIG.25, the sliding window comprises four switches. That is, four switchesare turned on at any given time. Therefore, the Nth exclusive OR gatehas, as its two inputs, the output of the Nth OR gate and the output ofthe N−4 OR gate. In like manner, each of the exclusive OR gates in thechain is coupled to the output of its companion OR gate and to theoutput of the OR gate which is four OR gates higher in the daisy chain.

For the sake of illustration, an input 1 is coupled into the OR gateN+1. All the OR gates have pull down resistors, or similar mechanisms,such that when a “1” is not coupled into the OR gate's input, the inputremains in a low or “0” condition. The output of the OR gate N 2501 isthe OR of one input (which is a 0) and a second input to the Nth OR(gate 2501) (which is also a 0). Therefore the output of OR gate 2501 isa 0. The 0 from the output of OR gate 2501 is coupled to the input ofthe Nth exclusive OR gate 2503. The output of the N−4 OR gate is alsocoupled into the input of exclusive OR gate 2503. The two inputs to theexclusive OR gate are 0 and therefore the output of exclusive OR gate2503 is 0. OR gate 2505 has as one input a 1. This 1 marks the locationof the beginning of the sliding window of switches that will be turnedon. The output of OR (gate 2501) is a 1 and is coupled into an input ofexclusive OR gate 2507. The other input of exclusive OR gate 2507 is theoutput of the OR gate N−3 which is 0. Because the two inputs toexclusive OR gate 2507 are different, the output is equal to 1.Similarly, the one which was inserted into OR gate N+1 is coupledthroughout the entire OR gate chain. Therefore, all OR gates after theOR gate N+1 2505 have as their output a 1.

FIG. 26 is a circuit diagram of an eight segment programmable gainattenuator ladder. The attenuator ladder comprises eight sections 2601,2603, 2605, 2607, 2609, 2611, 2613 and 2615. Each of the segmentscomprises four taps. Section 2617 represents the termination resistors.FIG. 26 represents an actual implementation of a fine programmable gainattenuator 16.

FIG. 27 is a schematic of the upper level of one of the segments, forexample, 2601, as illustrated in FIG. 26. Segment 2701 comprises fourswitches, 2703, 2705, 2707 and 2709.

FIG. 28 is a schematic diagram representing the lower half of thedifferential segment, such as 2601. The segment illustrated at 2801comprises four switches, 2803, 2805, 2807 and 2809.

FIG. 29 is a graph of the frequency response of a programmable gainattenuator according to embodiments of the invention. The graphillustrates 32 curves corresponding to 32 taps of the programmable gainattenuator. The frequency response is a response in which the slidingwindow of the programmable gain attenuator is four switches. In thepresent example, the sliding window does not have extra switches at theend of the resistive ladder. The result is curve 2901 representing thecase in which the very last tap of the programmable gain attenuator isactive and only one switch is on. Curve 2903 represents a curve in whichthe last two switches of the programmable attenuator are on. Thedifference in resistance between a system having additional switches atthe end of the resistive ladder, results in the markedly different curveshapes as illustrated. In the case of curves 2901 and 2903, thebandwidth rolls off sooner than any of the other curves. Whether thebandwidth rolloff illustrated in curves 2901 or 2903 is significantdepends on the application in which the PGA is found. If is criticalthat the curves match closely, then it may be advantageous to addadditional switches at the end of the resistive steps ladder to keep theresistance steps of the sliding window constant.

FIG. 30 is a graph of the programmable gain attenuator step size versusthe step. In the graph of FIG. 30, point 3001 is the pointrepresentative of four switches being on. Point 3003 represents twoswitches being on and point 3005 represents one switch being on. FIG. 30illustrates the discontinuity in step size experienced by not havingswitches at the end of the resistive ladder. If such a discontinuity isundesirable, then implementation may include extra switches at the endof the resistive network.

FIG. 31 is a block diagram further illustrating programmable gainamplifier 214. The programmable gain amplifier 214 comprises threeparts, a coarse PGA 14 coupled to a fine PGA 16, coupled to a four timesgain amplifier 3201. The coarse PGA has a four-bit gain control. Thefine PGA has a five-bit gain control. The response of the overall coarseand fine PGA programmable gain attenuator is illustrated in FIG. 32.

FIG. 32 is a graphical plot of the coarse and fine steps of theprogrammable gain attenuator. Graph line 3301 represents the attenuationsteps of the coarse programmable gain attenuator. There are 16 discretesteps of programmable attenuation available. Graph line 3303 representsthe attenuation steps of the fine programmable gain attenuator. The fineprogrammable gain attenuator comprises 32 steps corresponding to itsfive-bit gain control. The coarse and the fine PGA are of differentconfigurations. The schematic of the coarse, 4 db per step section, PGAis as seen in FIG. 13. This is necessary because the coarse PGA may havesignificantly large voltage swings coupled into its input. Because ofthe large voltage swings the input stage which receives the input signalmay comprise one or more sections of PGA as illustrated in FIG. 11. This“super coarse” section may be followed by sections as illustrated inFIG. 5 and FIG. 13. FIG. 5 and FIG. 13 together may comprise the overallPGA. This coarse gain section is followed by a 1 db per step section.Although the serial arrangement of the coarse and the fine PGA isarbitrary in an equivalent electrical sense, from a practical standpointby placing the coarse PGA first, the signal to the fine PGA may bereduced to the point where techniques not appropriate for the circuitryof the coarse PGA can be applied to the fine PGA. The fine PGA,accordingly, accepts significantly reduced voltage swings when comparedwith the coarse PGA and, therefore, a sliding window approach, asillustrated in FIG. 22, may be utilized.

Linearity in the coarse PGA is achieved in part by eliminating thesignal path through the switch steps. In the fine PGA, however,linearity is achieved through the sliding window approach, which isviable because of the lower signal levels which travel through the finePGA. The overall response of the coarse and fine PGA is the sum of thegain settings as illustrated in the graph of FIG. 33. The fine PGAprovides 32 steps of approximately 0.2 dB and the coarse PGA effectivelyprovides 16 steps of approximately 1 dB. Both PGAs are controlled by anautomatic gain control circuit 220.

FIG. 33 is a block diagram of an automatic gain control (AGC) accordingto embodiments of the invention. Automatic gain control 220 controls thesetting of both the coarse programmable gain attenuator 16 and the fineprogrammable gain attenuator 14. The automatic gain control 220 is adigital control loop in which the signal level at the output of the PGA214 (as represented in the A-D FIFO 218) is compared to a setpoint 222.

When the gigabit transceiver system is turned on, the fine PGA 14 is setto a midpoint value by the automatic gain control 220. The automaticgain control 220 then goes through a process which sets the coarse PGA16 to a setting such that a signal in an acceptable range is receivedinto the A-D FIFO 218. In the present illustrative embodiment of theinvention, the coarse PGA is then maintained at the setting and all.adjustments in the signal level are accomplished through the use of thefine PGA 14. It is the function of the AGC circuit to keep level of theoverall signal coupled into the, A-D converter 216 nearly as constant aspossible.

The gigabit signal that will be received by the gigabit transceiver is acomplex signal. It is advantageous to pass the gigabit signal throughthe PGA and into the A-D without having the signal clip, that is withoutthe received signal being so large that it exceeds the input range ofthe AID 216. If the signal does clip, then errors will be introduced inthe received data stream. It is a characteristic of the gigabit signal,however, that the peak values occur only sporadically. For example, atypical gigabit signal may exhibit a peak value only once in 10¹⁵samples. Although once in 10¹⁵ samples is a large number, a typicalrequirement of the overall receiver is one error in 10¹⁵ samples.Therefore, a clipping rate of one in 10¹⁵ may be too high, as it mayconsume the entire error tolerance of the system. On the other hand, itis advantageous to utilize most of the range of the A-D converter 216 inorder to achieve the best resolution possible of the A-D 216.Accordingly, if the set point of the AGC is too low, the effectiveresolution of the signal is decreased.

It, however, is very difficult to control the level of the signal usingthe peak values, because the peak values occur so infrequently.Therefore, another method of control for the automatic gain control 220may be advantageous. In the present embodiment of the invention, thelevel of the automatic gain control 220 may be controlled by using theRMS (Root Mean Squared) value of the signal, because the ratio of theRMS value to the peak value of the signal is essentially a constant, butmay vary somewhat depending on such factors as the length of the cablelinking the gigabit transceiver to the gigabit receiver.

To determine the RMS value of a signal, it is typical to square thevalue of the signal and to compute its average value over a suitableperiod of time. This procedure can be used in embodiments of theinvention, but requires significant computing power in the form of amultiplier to square the value of the signal. Instead, however, theaverage absolute value of the signal is directly related to the RMSvalue, assuming that the distribution of the signal is filed. A Gaussiandistribution yields a reasonable approximation of the distribution ofthe gigabit signal. Using a Gaussian distribution, the ratio of theaverage absolute value of the signal to the RMS value has beendetermined by simulation to be 0.7979. Using this result, a target valuecan be set for the expected absolute value. The target value is set sothat the peak value of the signal is near to the full range of the A-Dconverter.

The coarse PGA is adjusted during the start-up process and is thenfrozen. No further adjustments to the coarse PGA occur until the systemis restarted. During start-up, the fine PGA is maintained at a centervalue while the coarse PGA is adjusted. When the start-up process iscomplete the coarse PGA is frozen and any changes in signal level areaccounted for by the fine PGA. The purpose of the adjustment of the finePGA is to account for any small changes in the signal resulting fromsuch causes as temperature change within the environment.

FIGS. 33 and 34 illustrate an AGC system as may be used to control PGAssuch as those described above.

FIG. 33 illustrates the functioning of the automatic gain control,according to an embodiment of the invention. A coarse gain controloutput register 3321 provides a four bit gain control for the coarsePGA. A fine gain control output register 3328 provides five bits ofcontrol for the fine gain PGA. The notation associated with the coarsegain, e.g., U4.0, indicates the number of bits as well as what portionof the total number of bits that are fractional. Therefore, the notationU4.0, of the coarse gain control, indicates an unsigned four bitquantity with zero fractional part. In contrast, the input to absolutevalue block 3301 has a notation of S8.7. The S8.7, indicates that thequantity is a signed quantity and that the fractional portion of thatquantity is 7 of the 8 bits.

Absolute value block 3301 accepts a sample from the A-D FIFO 216, whichis in the receive clock domain. A second clock domain comprises theanalog clock domain, which is used, for example, to sample the inputsignal at the line interface 210. By accepting the signal from thereceive clock domain, automatic gain control 220 bridges the gap betweenthe analog sampling domain and the receive clock domain. The receiveclock domain is asynchronous with respect to the input sampling clockdomain. For this reason, the A-D FIFO 216 is used to couple one clockdomain to another without losing data.

In absolute value block 3301 the absolute value of the accepted signalis taken. The sign bit is thereby eliminated. Therefore, the output ofthe absolute value block 3301 is an unsigned 7 bit number represented bythe notation U7.7. Block 3305 in combination with block 3303 form anaccumulator circuit. The accumulator circuit accumulates values from theabsolute value block 3301 over 128 cycles. Once 128 cycles have beenaccumulated, the accumulated value is then provided to block 3307 andthe accumulated value, represented in block 3305, is cleared. In otherwords, blocks 3301 and 3305 define an accumulate and dump filter. Whenthe AGC process is started, the accumulate and dump filter is initiallycleared. The accumulate and dump filter will then accumulate a valueover 128 clock cycles. Once the accumulate and dump filter has operatedover the 128 cycles, the accumulated value will be transferred toregister 3307, and a new accumulation cycle will begin. Because register3307 is loaded only once every 128 clock cycles, it is clocked at 1/128of the receiver clock frequency. In the present exemplary embodiment,the symbol rate from the receive clock is 125 MegaHertz (MHz).Therefore, the clocking of values into and out of register 3307 takesplace at a clock frequency equal to 125 MHz divided by 128 orapproximately 1 MHz. As a consequence, the remainder of the AGC needonly run at a 1 MHz rate. The output of the register 3307 is arepresentation of the accumulated absolute value of the signal. Theoutput of register 3307 should be equal to the reference level 331,which is equivalent to a setpoint 222 of the automatic gain control. Inprinciple, the function of the AGC is to change the number appearing inregister 3307 such that it is as close as possible to the referencelevel 3311. The difference between the reference level 3311 and theoutput of register 3303 is computed in block 3309. The output of block3309 represents an error signal defining the difference between thereference level and the average absolute value of the gigabit signal.The reference level coupled into the AGC at 3311 in the presentembodiment is a number found by simulation (as discussed previously).The error signal at the output of block 3309 ideally will be zero. Inpractice the error value is always some non-zero value. The error valuefrom the output of block 3309 is then scaled in block 3315. Block 3313selects the value to multiply by the error signal. In the presentembodiment, block 3313 can provide either a one times or a four timesmultiplication depending on the value of its select line. The selectline of block 3313 is represented in FIG. 33 by the input line labelCagchigear. The error value multiplied by the selected amplificationfactor is then coupled into the accumulator circuit comprisingcomparison block, multiplexer 3319, and coarse gain control register3321. The circuit comprising blocks 3317, 3319 and 3321 form anintegrator, which integrates the error signal. This integrator circuitis used to control the coarse gain PGA, thereby forming a feedbackcontrol loop. Similarly, the error signal output from block 3309 iscoupled into the integrator circuit comprising blocks 3323, 3325 and3328. The fine gain AGC does not include the scaling factor provided byblock 3315 to the coarse AGC. Additionally, the fine gain controlregister 3328 represents five output bits as opposed to the four outputbits of the coarse gain control register. These two factors contributeto the fact that the fine gain control loop has a slower response. Thefine gain loop is also a more precise loop, having one more bit ofresolution.

Initially, the coarse gain AGC is converged by being operated for aperiod of time. During the period that the coarse AGC is being operated,the fine gain AGC is set to a midrange value, and the fine AGC controlremains reset. Once the coarse gain AGC has converged to a value, thevalue is frozen and the fine gain AGC is then activated. The fine gainAGC then provides control of the AGC loop.

The multiplying factor provided to the coarse gain AGC loop throughblock 3315 can be used to hasten the convergence of the coarse gain AGCloop. Initially, when the coarse gain AGC is turned on, themultiplication factor can be set to the higher value, in this case four,in order to speed the convergence initially of the coarse gain loop.Once the initial portion of the convergence has taken place, the gainfactor can be switched to the lower gain factor, in this case one, inorder to achieve a more precise convergence.

There are multiple ways to compute the peak-to-RMS ratios of a signalsuch as used with embodiments of the present invention. In the case ofthe present invention, the peak-to-RMS ratios used have been computedexperimentally through the use of simulation. The absolute peak value ofthe signal is fairly easy to compute, but it is too pessimistic becausethe probability of reaching it may be orders of magnitude lower than thespecified error rate. For example, if the specified error rate is one in10¹⁵, using the absolute peak value of the signal may result in an errorrate as low as one in 10³⁰. By setting the PGA so that the gigabitsignal never exceeds the input range of the A/D 216, a very low errorrate is achieved, but the attenuation of the PGA is so high that signalresolution is sacrificed. The specified error rate (SER) is the errorrate at which errors are produced at an acceptable level for theoperation of the system. An assumption is made that, although clippingof the input signal is undesirable, sporadic clipping is relativelyharmless if its probability is much lower than the SER.

Therefore, the present computation proceeds with the assumption that theprobability of sporadic clipping is to be kept lower than the SER, buthigher than the probability of error if the absolute peak value of thesignal were used.

To compute the probability of clipping at a certain level, theprobability density function (PDF) of the signal may be firstascertained, then the clipping level can be set such that theprobability of clipping is sufficiently low, for example, 1 in 10¹⁵. Forthis purpose, a Gaussian-type distribution function was examined todetermine if the Gaussian distribution could approximate the PDF of thegigabit signal sufficiently to be used in lieu of the PDF function ofthe gigabit signal. It was found that a Gaussian approximation is notsufficient because the critical part of the probability densityfunction, in this case the tail, is not sufficiently represented by theGaussian approximation. In other words, the trailing portion of theprobability density function, which is integrated in order to find theprobability of exceeding a certain magnitude, is not well approximatedby a Gaussian distribution. It was found, through simulation, that abetter approach is to use a bound for the tail of the probabilitydensity function, such as the Chernoff Bound. The Chernoff Bound can becomputed relatively easily based on the impulse response of the gigabittransmission and echo paths, and can be used to provide an accurateestimate of the probability of clipping. A program named Peak Bound waswritten to compute the peak-to-RMS ratios and to set the target value ofE{|x|}. The Chernoff bound is represented below.P(X>x)≦e ^(−sx) φX(s)  equation 1.

In the Chernoff Bound equation, the first term P(X>x) indicates that theprobability of the PDF function being greater than a certain value x,which in this case has been set to 10⁻¹⁵, is less than or equal toe^(−sx)φX(s). The approximation turns out to be accurate and so can be,for practical purposes, written as an equals type equation instead ofless than or equal, as shown in equation 2 below.P(X>x)=e ^(−sx) φx(s)  equation 2

ΦX(s) is the Fourier transfer of the probability density function. Thecharacteristic function can be computed based on the impulse response ofthe gigabit transmission cable. Computation of the characteristicfunction is well known in the art. The term S within the Chernoff Boundequation is a number that is adjusted, in the present computation, inorder to achieve the tightest possible bound. The tightest possiblebound is equivalent to the lowest probability. The value of S can befound through numerical methods.

What is claimed is:
 1. An apparatus for providing an adjustablebandwidth high pass filter, the apparatus comprising: a high-pass filterhaving an input capacitor in series with a resistive ladder having aplurality of resistors coupled in series, the coupling between thecapacitor and a first resistor of the resistive ladder defining a firsttap and successive couplings between resistors forming successive taps,the last resistor of the resistive ladder being coupled to a ground; aplurality of bandwidth-adjusting resistors, each having a first endcoupled to the first tap; and a plurality of switches, each connectedbetween a second end of a corresponding one of the bandwidth-adjustingresistors to a ground.
 2. The apparatus as in claim 1, wherein theground comprises an AC ground providing a DC bias.
 3. The apparatus asin claim 1, wherein the switches comprise semiconductor switches.
 4. Theapparatus as in claim 3, wherein the semiconductor switches compriseMOSFETs (Metal Oxide Semiconductor Field Effect Transistors).
 5. Theapparatus as in claim 4, wherein the MOSFETs comprise N-type MOSFETs. 6.The apparatus as in claim 4, wherein the MOSFETs comprise P-typeMOSFETs.
 7. The apparatus as in claim 1, wherein at least one of theplurality of switches comprises a MOSFET device having a source, a gateand a drain, the source coupled to the second end of one of thebandwidth-adjusting resistors, the drain of the MOSFET device coupled tothe ground and the gate being coupled to a control circuit.
 8. Theapparatus as in claim 7, wherein the control circuit comprises anamplifier output coupled to the gate of the MOSFET device.